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Financial Instruments	9 Months Ended
Financial Instruments	Sep. 30, 2017
Financial Instruments [Abstract]
Financial Instruments	7. Financial Instruments The fair value of financial instruments is determined by reference to observable market data and other valuation techniques as appropriate. The only category of financial instruments where the difference between fair value and recorded book value is notable is long-term debt. The estimated fair value of total long-term debt, excluding capitalized lease obligations, was $24,199 million at September 30, 2017, and $27,968 million at December 31, 2016, as compared to recorded book values of $23,523 million at September 30, 2017, and $27,707 million at December 31, 2016. The fair value of long-term debt by hierarchy level at September 30, 2017, is: Level 1 $24,021 million; Level 2 $172 million; and Level 3 $6 million. Level 1 represents quoted prices in active markets. Level 2 includes debt whose fair value is based upon a publicly available index. Level 3 involves using internal data augmented by relevant market indicators if available.